Ultraviolet irradiated herpes simplex virus (HSV) has been shown to transform cells in tissue culture in respect to (1) the addition of viral gene for thymidine kinase to cells, (2) to cause cells to synthesize HSV virus antigens and (3) to induce cells to form tumors in animals into which they have been injected. The studies proposed in this grant application attempt to determine which virus genes must function in order for HSV to transform cells and which HSV antigens are associated with the transformed state. We shall also determine the effect of HSV transformation on the capacity of L cells to form progrssive tumors when injected into mice. These cells in the absence of HSV trnasformation show a very low tumorigenic potential. We also will characterize a class of HSV mutants in which the infectivity is inactivated by one cycle of freezing and thawing.